Glycolysis

The Glycolytic Pathway

The Reactions of Glycolysis

Fermentation: The Anaerobic Fate of Pyruvate

Control of Metabolic Flux

Metabolism of Hexoses Other Than Glucose

The Glycolytic Pathway(Embden-Meyerhof-Parnas Pathway)

Glycolysis converts one C6 unit (glucose) to two C3 units(pyruvate) of lower energy in a process that harnesses thereleased free energy to synthesize ATP from ADP and Pi

Overall reaction -

Glucose + 2NAD+ + 2ATP + 2Pi →2NADH + 2pyruvate + 2ATP + 2H2O + 4H+

Stage I - Investment of 2ATP to split hexose glucose into 2molecules of triose glyceraldehyde-3-phosphate

Stage II - Generation of 4ATP from the conversion ofglyceraldehyde-3-phosphate into pyruvate

Glycolytic enzymes located in cytosol, loosely associated,no organized complexes

Oxidizing power of NAD+ must be recycled

1. Anaerobic muscle - homolactic fermentation2. Anaerobic yeast - alcohol fermentation3. Aerobic conditions - mitochondrial oxidation

Reactions of Glycolysis

Hexokinase (glucokinase in liver)

phosphoryl group transfer - first ATP investment

Random Bi Bi mechanism

ternary complex with glucose-Mg2+-ATP (catalysis byproximity effects)

Reactions of Glycolysis

Phosphoglucose isomerase (glucose-6-phosphateisomerase)

isomerization (aldose to ketose) reaction

pH dependent, pK = 6.7 (Glu) and pK = 9.3 (Lys)

absolute stereospecificity

Reactions of Glycolysis

Phosphofructokinase

phosphoryl group transfer - second ATP investment

one pathway rate-determining reaction

regulated enzyme

Reactions of Glycolysis

Aldolase

retro aldol condensation

Uni Bi kinetics

stereospecificity

two mechanistic classes:

Class I - Schiff base formation-enamine stabilization

Class II - Divalent cation stabilization of enolate

The Reactions of Glycolysis

Triose phosphate isomerase

isomerization reaction

concerted general acid-base catalysis involving low-barrierH-bonds

pH dependent - pK = 6.5 (Glu, His) and pK = 9.5 (Lys)

loop structure gives stereoelectronic control

diffusion-controlled reaction (catalytic perfection)

Reactions of Glycolysis

Glyceraldehyde-3-phosphate dehydrogenase

aldehyde oxidation drives acyl-phosphate synthesis - firsthigh-energy intermediate

NAD+ reduction

nucleophilic SH group forms thioester bond

Reactions of Glycolysis

Phosphoglycerate kinase

phosphoryl transfer - first ATP generation

sequential kinetic mechanism

two-domain enzyme (catalysis by proximity effects)

driving force of reaction is phosphoryl group transfer

The Reactions of Glycolysis

Phosphoglycerate mutase

transfer of functional group from one position to another ina molecule

phosphoenzyme (His phosphorylated)

formation of bisphospho intermediate (2,3-bisphosphoglycerate)

detour pathway in erythrocytes (Hb allostery)

Reactions of Glycolysis

Enolase

dehydration reaction - second high-energy intermediate

divalent cation required (Mg2+)

Reactions of Glycolysis

Pyruvate kinase

phosphoryl transfer reaction - second ATP generationK+ and Mg2+ required

Fermentation: The Anaerobic Fate of Pyruvate

Need to recycle NAD+

Homolactic fermentation

Glucose + 2ADP + 2Pi→ 2lactate + 2ATP + 2H2O + 2H+

∆G˚' = -196 kJ.mol-1

Lactate dehydrogenasepyruvate + NADH → lactate + NAD+

stereospecificity in hydride transfer

mammalian isozymes:

two subunits (M and H) - five tetrameric forms

H4 LDH has low Km for pyruvate and is allostericallyinhibited by pyruvate

M4 LDH has higher Km for pyruvate is not inhibited

Fermentation: The Anaerobic Fate of Pyruvate

Need to recycle NAD+

Alcoholic fermentation

Glucose + 2ADP +2Pi→ 2ethanol + 2CO2 + 2ATP + 2H+

∆G˚' = -235 kJ.mol-1

Pyruvate decarboxylase - thiamine pyrophosphatecoenzyme (not present in animals)

Alcohol dehydrogenase - Zn2+ and NADH dependent

Control of Metabolic Flux

Rate of flow (flux = J) of intermediates through a metabolicpathway is constant and is set by the rate-determiningstep(s)

But the pathway must be able to respond to specificbiological energy needs (i.e., communicate with othersteps)

J = vf - vr

S AJ

B PJvf

vrrate-determining step

∆ ∆J

J

v

v vf

f r

= ⋅−

[ ][ ] ( )

AA

Two cases:

1. Irreversible reaction - vr approaches 0, vf (vf - vr)approaches 1, nearly equal increase in ∆[A] to respond toincrease ∆J

2. Approaching equilibrium - vr ~ vf, vf/(vf - vr)approaches infinity, much smaller increase in ∆[A] torespond to increase ∆J

Control of Metabolic Flux

Rate determining step functions far from equilibrium andhas a large negative free energy

Substrate control is only one way to rationalize control ofrate-determining step of a metabolic pathway

Other flux-controlling mechanisms:1. Allosteric control - regulated by effector molecules

(substrates, products, coenzymes in the pathway) thatchange enzyme activity

2. Covalent modification - regulated by modifications(phosphorylation, dephosphorylation) that changeenzyme activity

3. Substrate cycles - vf and vr of nonequilibrium reactions

are catalyzed by different enzymes and thus may beindependently varied

4. Genetic control - enzyme concentration may be alteredby protein synthesis in response to metabolic needs

Mechanisms 1-3 respond rapidly (seconds to minutes) anddenoted short-term controlMechanism 4 responds more slowly (hours to days) anddenoted long-term control

Control of Metabolic Flux

Control of glycolysis in muscle

Look for large negative ∆G under physiological conditions:

hexokinase ∆G = -27 kJ.mol-1

phosphofructokinase ∆G = -26 kJ.mol-1

pyruvate kinase ∆G = -14 kJ.mol-1

Phosphofructokinase (PFK-1):

Tetrameric enzyme (R and T states)ATP is substrate and allosteric inhibitorTwo ATP binding sites per subunit (substrate site andinhibitor site)ATP binds well to substrate site in either R or T stateATP binds to inhibitor site in T stateFructose-6-phosphate binds to R state

At high [ATP], ATP acts as allosteric inhibitor anddecreases affinity of PFK-1 for F6P

More important allosteric effector is fructose-2,6-bisphosphate

Control of Metabolic Flux

Control of glycolysis in muscle

Metabolic flux through glycolysis can vary 100-fold butATP varies only 10%Adenylate kinase - 10% decrease in [ATP] translates into a4-fold increase in [AMP]

Consider substrate cycling:

Two enzymes are involved in establishing equilibrium-likeconditions:

1. Phosphofructokinase-1 (PFK-1)fructose-6-phosphate + ATP

→ fructose-1,6-bisphosphate + ADP∆G = -26 kJ.mol-1

2. Fructose-1,6-bisphosphatase (FBPase)fructose-1,6-bisphosphate + H2O

→ fructose-6-phosphate + Pi

∆G = -9 kJ.mol-1

Net reaction is ATP + H2O ⇔ ADP + Pi (futile cycle)

Control of Metabolic Flux

Control of glycolysis in muscle

Assume 4-fold increase in [AMP] causes PFK-1 activity(vf) to increase from 10 to 90% of its maximum and FBPaseactivity (vr) to decrease from 90 to 10% of its maximum

Maximum activity of PFK-1 is 10-fold > maximum activityof FBPase

Assume PFK-1 activity = 100 units (vf)FBPase activity = 10 units (vr)

At low [AMP]:Jlow = vf(low) - vr(low) = 10 - 9 = 1

At high [AMP]:Jhigh = vf(high) - vr(high) = 90 - 1 = 89

Therefore:Jhigh/Jlow = 89/1 = 90!

Laws of thermodynamics are not violated!

(Cannot favor both forward and reverse reactions of asingle enzyme)

Metabolism of Hexoses Other Than Glucose

Fructose, galactose, and mannose are converted toglycolytic intermediates and then processed as describedpreviously

Fructose - fruit and hydrolysis of sucrose

In liver:

Fructokinase - phosphoryl transfer to form fructose-1-phosphate

Fructose-1-phosphate aldolase (type B) - aldole cleavage toform dihydroxyacetone phosphate and glyceraldehyde

Glyceraldehyde kinase - phosphoryl transfer to formglyceraldehyde-3-phosphate

or

Alcohol dehydrogenase, glycerol kinase, glycerolphosphate dehydrogenase

Excess fructose depletes liver Pi (activating glycolysis →lactate buildup)

Metabolism of Hexoses Other Than Glucose

Galactose - hydrolysis of milk sugar(not recognized by glycolytic enzymes)

Galactokinase - phosphoryl transfers to form galactose-1-phosphate

Galactose-1-phosphate uridylyl transferase - uridylyltransfer from UDP-glucose to galactose-1-phosphate

UDP-galactose-4-epimerase - epimerization converts UDP-galactose to UDP-glucose

Phosphoglucomutase - isomerization reaction to formglucose-6-phosphate

Galactosemia - increased [galactose] and [galactose-1-phosphate] → galactitol in lens of eye

Metabolism of Hexoses Other Than Glucose

Mannose - digestion of polysaccharides and glycoproteins

Hexokinase - phosphoryl transfer to form mannose-6-phosphate

Phosphomannose isomerase - isomerization to formfructose-6-phosphate (mechanism similar tophosphoglucose isomerase)